This invention relates in general to control mechanisms for use with toner monitors of electrographic reproduction apparatus, and more particularly to a toner monitor control mechanism for selectively adapting a reflective type toner monitor to effect regulation of the concentration of toners of a variety of colors and black.
In typical electrographic reproduction apparatus, it is common practice to modify a uniform charge on a dielectric member to form a latent electrostatic image corresponding to information to be reproduced. Such latent image is then developed by bringing it into contact with a developer material. Developer materials generally comprise a two-component mixture of granular carrier particles and pigmented or dyed resin-based electroscopic marking particles, referred to as toner, which triboelectrically adheres to the carrier particles. When the developer material contacts the latent electrostatic image on the dielectric member, the toner separates from the carrier particles and is deposited on the latent image in an imagewise configuration to form a developed image. Such developed image is thereafter transferred to receiver material and permanently fixed thereto, by heat and/or pressure for example, to form the copy of the information to be reproduced.
As will be readily appreciated, during such image development process, toner is continuously being depleted from the developer material. Since the density of the developed images being formed is directly related to the concentration (relative amount) of toner in the developer material, such concentration must be regulated within a given range in order to avoid an undesirable reduction in image density which would result in the production of unacceptable copies. One well known method for regulating toner concentration is to optically monitor the toner concentration and add toner to the developer material when the concentration drops to a preselected level below which unacceptable copies will be produced. An apparatus employing this method is described in U.S. Pat. No. 3,876,106 (issued Apr. 8, 1975 in the name of Powell et al). In such apparatus, light is directed at the developer material and the amount of light reflected therefrom is measured. The toner, which is usually black, is highly light absorbing and thus reflects substantially less light than the carrier particles. Accordingly, the reflectivity of the developer material (i.e., the amount of light reflected therefrom) depends upon the relative proportions of the components of the developer material. A processor, having a stored program, produces a signal in response to the amount of reflected light and in accordance with such program. The produced signal corresponds to the corresponding proportion of toner in the developer material (i.e., the toner concentration). This signal can then be used to activate a toner replenisher when the signal indicates that toner concentration is at a preselected minimum acceptable level.
It has recently been proposed to provide an electrographic reproduction apparatus with interchangeable developer stations respectively containing toner of different colors (e.g., red, green, blue, cyan, magenta, yellow or black). By interchanging such developer stations, copies may be selectively reproduced in different colors. Alternatively, copies may be reproduced containing multiple colors for the purpose of accenting or highlighting certain areas of the copies. The use of toners of different colors introduces a significant complexity in regulating toner concentration. Such complexity results from the fact that the different colors cause the toners to reflect light differently. For example, light colored toners are highly light reflective relative to carrier particles, as opposed to black toner which as noted above is highly light absorbing. Thus the reflectivity of the developer material containing light colored toner behaves substantially differently from developer material containing black toner.